A battery pack containing a secondary battery, particularly a lithium-based secondary battery, includes a safety protection circuit provided within the battery pack. The safety protection circuit is provided with an interrupter unit that interrupts the secondary battery from a discharging circuit or a recharging circuit when an abnormal situation occurs.
In the safety protection circuit that detects over-discharging or temperature rise and interrupts the secondary battery, it is typical that the interrupter unit automatically restores the original state as soon as the abnormal situation is released (see, e.g., Japanese Patent Laid-open Publication No. 10-144285).
In case the battery pack is used as a power source of an electric power tool, however, there occurs a problem if the restoration proceeds automatically regardless of the user's intention. In other words, if the restoration occurs in a state that the switch of the electric power tool is turned on, the electric power tool starts its operation at that moment. This may sometimes give rise to a very dangerous situation.